If you don't have a Flickr pro account, you'll know that, while Flickr stores your pictures at full res, it serves them smaller. Say you have a 2592x1944 original, the largest you can download is 1024x768.
If you want to download the original file, you can. Just set up an RSS feed of your photostream, and open your RSS reader. Find the photo you want. You'll see the words "original enclosure" in the feed item. This is the link to the full-res file. Download and you're done.
DOWNSIDE: You will only see photos you've tagged for the public.
UPSIDE: But this means you'll be able to go back further in time than your 200 limit if you have more than 1 photos tagged private.